The present invention relates in general to tuning circuits, and more particularly, to an active impedance tuning circuit for compensating the loss of power between electrically mismatched source and utilization circuits whereby the power available to the latter is increased.
A common problem occurring between electrically mismatched source and utilization circuits is the loss of power due to the reflection of the input signal incident to the junction of the source and utilization circuits. The reflected signal may include multiple harmonics of the incident signal as well as the fundamental frequency shifted in phase depending upon the nonlinearity across the junction, causing the reflected wave to interfere with the forward propagating input signal thereby developing a power loss in the forward transmission path limiting the power available to the utilization circuit. This is true especially for input signals operating at high frequencies, i.e., in the ratio frequency (RF) band or higher. The reflected waveform, and correspondingly the power loss, may be reduced by matching (equating) the impedances of the source and utilization circuits observed on both sides of the junction. Conventional impedance matching techniques include the use of passive components serially coupled in the forward transmission path selected such that the source and utilization impedances are matched at the fundamental frequency. However, the impedance of the passive components in the forward transmission path also dissipates power again reducing the power transfer even for a matched condition. In addition, passive components such as fixed inductors and capacitors and stub tuners are frequency dependent devices and therefore provide satisfactory impedance matching only over a limited bandwidth.
In broad band applications, impedance matching may also be required at certain harmonic frequencies of the fundamental, typically the second and third harmonics, to achieve maximum power transfer. Computer controlled impedance tuners are available for tuning over the broad band, but typically are adjusted at the fundamental frequency and do not provide independent tuning of the reflected harmonic components. High efficiency amplifiers are also known for increasing the power available to the load; however, such devices usually require custom design and fabrication for each specific application and become increasingly difficult to use at higher frequencies.
Hence, there is a need for an active impedance tuning circuit for increasing the power available to the latter of electrically coupled, possibly mismatched source and utilization circuits, independent of the operating frequency.